A switching regulator generates internal power supply voltage which is used by a load circuit, from power supply voltage which is supplied. A switching regulator which generates DC internal power supply voltage from a DC power supply voltage is also called a “DC-DC convertor”.
A switching regulator has a first transistor which is connected to a higher power supply voltage side, and a second transistor which is connected to a lower power supply voltage (ground) side, and supplies the output voltage to a load circuit via an LC smoothing circuit connected to the connection nodes of both transistors. By appropriately controlling the switching operation of both transistors, the output voltage is maintained at a desired potential. The LC smoothing circuit is disposed outside an integrated circuit chip where the switching regulator is formed, or is disposed on the integrated circuit chip.
The switching regulator monitors the output voltage, and alternately turns the first transistor and the second transistor ON so that the output voltage is maintained at a desired potential. For example, if the load is heavy and the output voltage drops, the “ON” period of the first transistor is increased, so as to suppress a drop in the output voltage. Alternatively, the switching regulator monitors the output current and controls the “ON” period of the first transistor according to the level of the output current, which changes according to the level of the output load.
The switching regulator is disclosed in the Japanese Patent Application Laid-Open No. 11-155281, and the Japanese Patent Application Laid-Open No. 2009-148111.
In order to improve the efficiency of a switching regulator, switching counts of both transistors are decreased in a light load state. For example, the switching frequency is lowered in a light load state to decrease the switching counts. Alternatively, the switching counts are decreased for a certain period by stopping the switching operation of both transistors in a light load state. Thereby when the load is light, a switching loss generated with the switching operation is decreased so as to improve efficiency.
However, if a number of time of switching is decreased or a switching operation is stopped, a period when electric current is not supplied to the output is generated, and a drop in the output voltage during this time increases fluctuation (ripple) of the output voltage, which is a problem. Particularly if the load gradually increases even with a light load state, the degree of a drop in the output voltage also increases, and the output voltage fluctuation (ripple) also increases.